The present disclosure relates to the measurement of extremely weak magnetic signals.
To detect and monitor military equipment, for example communications terminals and land, air, sea, and space vehicles, at great distances, it is desirable to observe unintended signals that are created by the equipment and propagate away. Electromagnetic waves are among the signals that are useful for this purpose. To avoid detection, an item of military equipment may be designed with great care to minimize the likelihood that its unintended electromagnetic emissions will reveal its presence to adversaries.
In spite of the care taken to reduce such emissions, any equipment that is electrically operated or that incorporates moving structures containing electrically conductive, magnetized, or magnetically permeable materials will unavoidably create static or quasi-static magnetic fields and time-varying electromagnetic fields during operation. These fields may be faint even in the immediate vicinity of the equipment and in any event will attenuate with distance from the equipment. Nevertheless, weak electromagnetic signals caused by the equipment will exist at great distances. In principle, the magnetic component of these signals may be exploited using an extremely sensitive magnetometer to detect even equipment that has been painstakingly designed to minimize the unintended fields it produces.
The performance of a magnetometer system is limited by the total noise that contributes to its measurement. The sensor itself, and its natural and human environment, all contribute to the total noise, which determines the system resolution, in Tesla/√Hz. A magnetometer must have a resolution on the order of 10 fT/√Hz or better to detect magnetic signals from stealthy military systems at distances that permit effective surveillance or defense. Stated another way, the magnetometer must have input-referred noise on the order of 10 fT/√Hz or less to be useful in important military applications. The output of an ultra-low noise sensor can be numerically processed to observe a weak magnetic signal of interest it contains. The same result cannot be achieved using a conventional sensor, because the signal of interest would still be indistinguishable from noise after the output of the conventional sensor has been processed.
In addition to ultra-low noise or, equivalently, high resolution, a magnetometer system that is useful for locating and monitoring stealthy military systems requires portability for field use and the ability to operate without cryogenic cooling and in the presence of the relatively large magnetic field of the earth. Conventional magnetometers lack this combination of attributes. In particular, a sensor would need to occupy less than 100 cm3 to be sufficiently portable, while achieving the necessary resolution of 10 fT/√Hz. State of the art magnetometers using superconducting quantum interference device (SQUID) sensors can achieve the necessary resolution, but occupy more than 30,000 cm3 and require cryogenic cooling. Certain atomic magnetometers, for example spin-exchange relaxation-free (SERF) magnetometers, may be suitable for miniaturization but are saturated by the earth's magnetic field and therefore unable to measure weak magnetic fields and fluctuations in its presence.
For the foregoing reasons, there is a need for ultra-low noise magnetometers that are compact, unaffected by the earth's magnetic field and preferably able to operate without cryogenic cooling.